Starting on Diablo disk and DEC tape drive restorations

DIABLO DISK DRIVE RESTORATION BEGINNING

I opened the Diablo drive to inspect and begin the restoration. It has no heads installed, but I have a set to put in later. I cut a zip tie that secured the rotary actuator in position for transit. Overall everything looks undamaged and ready to be tested.

I have lots of experience with the Diablo since it is used in the Xerox Alto, which I restored along with Marc Verdiell and Ken Shirriff. As part of that effort, I engineered an FPGA controller and tested it on a Diablo of my own. I also worked with drives from several other Alto systems.

IBM created the single platter disk drive in the early 1960s, announcing it as the internal drive of the IBM 1130 and then as a 2310 peripheral for 1130 and S/360 systems. The disk has a single 14" platter of aluminum coated with an iron oxide surface. The platter is contained in a plastic cartridge that is slid into the front of a disk drive. 

There is a head for both surfaces of the platter, upper and lower, that moves radially from near the outer edge towards the center hub, stopping in 100 defined positions that represent one of 200 concentric circular tracks. Thus there are 400 tracks of information, 200 per head.

The disk rotates at 1500 rpm under the heads and has eight notches cut evenly around the periphery of the central hub, delineating eight physical sectors of the track. There is also a second notch at one of the positions, called the index mark, that defines the beginning of the track. The electronics in the drive ignores the odd numbered sector marks, thus the track is broken into four logical sectors.

Each sector is a fixed length, 321 words. Since the 1130 word size is 16 bits, the capacity of a sector is 642 bytes, 2,568 per track and just over 1MB per disk cartridge. For most purposes, the first word of a sector is used to hold the sector number, thus there are only 320 words the user can use. 

The data is recorded serially on the track with an alternating pattern of clock and data bits. Each bit is created by reversing the magnetic field. There is always a reversal for the clock bit since there will always be one received, but if the data bit value is to be written as zero then no reversal is made at that time. Thus we either have a reversal to read a data bit of 1 or nothing which signifies a zero.

After the 16 data bits of a word are written, four more bits that are error correction values, a form of parity, are written. These check each group of four bits in the word for validity. The beginning of a sector consists of a string of 0s (clock bit but no data bit) then a special format word to indicate that the next 20 bits is the first word of the sector. After the 321st word there are more zeroes written. 

The disk hardware will get into sync with the train of 0 words at the start of the sector so that it knows which magnetic reversal is a clock bit and which are data bits. The drive splits these out and sends clock pulses on one signal wire and data bits on another signal wire when the data value is 1. 

The disk hardware also sends the index marker pulse once per rotation and the four sector markers at the time of each of the four logical sectors on the track. It will switch to write mode if the control signal into the drive is set. It has another control signal that will activate the erase coil in the disk head, removing the previous magnetic reversals to allow for reliable clean writing to occur.

Writing uses a single line that has pulses for both clock and 1-valued data bits combined, unlike reading where the drive has separated clock and data. Thus, the drive is given the correct string of pulses to reverse the magnetic field of the write coil. 

There is a signal from the drive when the actuator is retracted to the position for the first track, called the home position. Moving the actuator is requested by control signals to the drive. The direction of movement is specified (inward towards track 200 or outward toward 1), the size of the movement which is one track or two, and an activation pulse to command one movement. 

The drive electronics spin the motor, load or unload the heads from the disk surface, report status from the drive, and implement the actions requested by control signals such as write or move. 

The drive is supplied with +15V, -15V, and 5V power from external supplies. 

Heads need to be aligned to a standard position such that a disk written on one drive can be moved to another drive and correctly read. This is done by sliding the heads on the actuator while reading a specially formatted disk alignment cartridge. 

A pattern is recorded on the pack at a given track position and nothing else is recorded nearby. Hooking the output of the read heads to an oscilloscope, the technician moves the heads until the signal is picked up. It becomes even and maximized at the correct position. The head is locked down with a setscrew and retested to verify its position. The two heads are done one after another. 

I don't have the special cartridge. If I can locate one I would buy it but they are pretty rare. Barring that, my restoration will require some clever workarounds to get the drive aligned so it can retrieve the data correctly from all the packs I own. 

Early tasks in the restoration are:

• Build FPGA based controller and connect to the drive
• Install new heads in approximate position
• Install terminator on drive
• Verify that drive spins up and loads heads properly
• Perform alignment somehow

WANT TO DO A QUICK CHECK OF THE DEC TSZ-07 9 TRACK MAG TAPE DRIVE

I kept a rack size 9 track tape drive, a DEC unit which is a rebadged Cipher Data drive, because it has a SCSI interface. That connects easily to my P390 server and appears to be a 3420 tape drive to the mainframe software. Since the P390 is already operational, this could be a quick project.

Unfortunately, I didn't find any schematics for this under either the DEC or Cipher Data names. There is a technical manual but it doesn't go down to the electrical connection level. This posed my first challenge because my normal practice would be to start with the power supply and verify it was producing clean, correct power before I connect it to the rest of the logic. 

The supply is a switcher made by GFC Power, a model GFXC 250-50, but again I can't find any schematics. That matters because there is a connector with some logic signals in addition to the power output connectors. I would prefer to bench test the supply but would need to make appropriate connections to the power supply logic connector. 

I have a few possible approaches:

• try to reverse engineer the logic connections
• just power up the entire drive without any prior testing
• disconnect the power output connectors and hook up load resistors to try to start the supply
• remove and test the capacitors, hope the rest of the power supply is good